Tires for all types of ground vehicles typically comprise a rubber composite, which is composed of a rubbery or elastomeric matrix with additives dispersed therein. Additives are needed to impart various desirable properties to a tire, including abrasion resistance (to reduce wear and tear), strength, stiffness, thermal conductivity (to dissipate heat effectively and efficiently), rolling resistance or wet grip, and chemical resistance. The use of an additive to enhance wear resistance must not in any way adversely impact on rolling resistance or wet grip. The words “rubber” and “elastomer” are hereinafter used interchangeably.
Rubbers or elastomers generally have a low thermal conductivity. Consequently, when the rubber generates heat through repetitive deformation, they store the generated heat that in turn raises the temperature of rubber itself, thereby promoting heat deterioration of the rubber. In an automotive tire that is subjected to repetitive deformation when the automobile is in motion, the generated heat must be rapidly released or dissipated. To achieve this goal, the heat conduction capability of a rubber may be improved by compounding a rubber with a filler having a heat conductivity higher than that of the rubber. However, in order to obtain a satisfactory effect, an excessive amount of thermally conductive additives is usually required and, as a result, the dispersion of the filler becomes non-uniform and dynamic properties are lowered. Furthermore, highest thermal conductivity materials are either too heavy (e.g. copper) or too expensive (e.g., carbon nanotubes). Metallic fillers are also corrosion-prone.
Carbon nanotubes (CNTs) do exhibit impressive strength, stiffness, and thermal conductivity and could be a good candidate additive for rubbers. However, attempts to produce CNTs in large quantities have been fraught with overwhelming challenges due to poor yield and costly fabrication and purification processes. Hence, even the moderately priced multi-walled CNTs remain too expensive to be used in high-volume applications or commodity products, such as structural polymer composites and tires. Further, for many applications, homogeneous dispersion of CNTs in a polymer and processing of polymer fluids containing a high CNT concentration have been difficult due to the tendency for CNTs to aggregate or physically entangle with one another and the chemical inertness of CNT surfaces.
Instead of trying to develop lower-cost processes for CNTs, the applicants sought to develop an alternative nanoscale carbon material with comparable properties that can be produced much more cost-effectively and in larger quantities. This development work led to the discovery of processes and compositions for a new class of nano material now commonly referred to as nano graphene platelets (NGPs), graphene nano sheets, or graphene nano ribbons [e.g., B. Z. Jang and W. C. Huang, “Nano-scaled graphene plates,” U.S. Pat. No. 7,071,258, Jul. 4, 2006].
An NGP is a platelet, sheet, or ribbon composed of one or multiple layers of graphene plane, with a thickness that can be as small as 0.34 nm (one carbon atom thick). A single-layer graphene is composed of carbon atoms forming a 2-D hexagonal lattice through strong in-plane covalent bonds. In a multi-layer NGP, several graphene planes are weakly bonded together through van der Waals forces in the thickness-direction. Multi-layer NGPs can have a thickness up to 100 nm, but typically less than 10 nm in the present application. Conceptually, an NGP may be viewed as a flattened sheet of a carbon nano-tube (CNT), with a single-layer graphene corresponding to a single-wall CNT and a multi-layer graphene corresponding to a multi-wall CNT. However, this very difference in geometry also makes electronic structure and related physical and chemical properties very different between NGP and CNT. It is now commonly recognized in the field of nanotechnology that NGP and CNT are two different and distinct classes of materials. Both NGPs and CNTs are also distinct from the conventional graphite nanoparticles.
NGPs are predicted to have a range of unusual physical, chemical, and mechanical properties and several unique properties have been observed. For instance, single-layer graphene (also referred to as single-sheet NGP) was found to exhibit the highest intrinsic strength and highest thermal conductivity of all existing materials, even higher than those of single-walled CNTs [C. Lee, et al., “Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene,” Science, 321 (July 2008) 385-388; A. Balandin, et al. “Superior Thermal Conductivity of Single-Layer Graphene,” Nano Lett., 8 (3) (2008) 902-907]. Single-sheet NGPs possess twice the specific surface areas compared with single-walled CNTs. The thermal conductivity of single-layer graphene, as high as 5,300 W/mk, is two times higher than the highest thermal conductivity of single-walled CNTs ever reported based on actual experimental measurements. Such a high thermal conductivity could translate into a great heat-dissipating capacity if NGPs are properly dispersed in a lubricant or grease material.
In addition to single-layer graphene, multiple-layer graphene platelets also exhibit unique and useful behaviors. Single-layer and multiple-layer graphene are herein collectively referred to as NGPs. Graphene platelets may be oxidized to various extents during their preparation procedures, resulting in graphite oxide or graphene oxide (GO) platelets. In the present context, NGPs refer to both “pristine graphene” containing essentially no oxygen (<1% by weight of oxygen, and preferably <0.05% by weight) and “GO nano platelets” of various oxygen contents. GO nano platelets produced by thermal exfoliation of heavily oxidized graphite typically have a C/O atomic ratio of <95/5 and more typically of <85/15. The term “slightly oxidized NGPs” refer to NGPs with a C/O ratio >95/5, which can be produced by exposing the pristine NGPs to an oxidizing environment for a controlled period of time. It is helpful to herein describe how NGPs are produced.
The processes that have been used to prepare NGPs were recently reviewed by the applicants [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. As illustrated in FIG. 1, the most commonly used process entails treating a natural graphite powder (referred to as Product (A) in FIG. 1) with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO) (referred to as Product (B) in FIG. 1). Prior to intercalation or oxidation, graphite has an inter-graphene plane spacing of approximately 0.335 nm (Ld=d002=0.335 nm or 3.35 Å, based on X-ray diffraction data readily available in open literature).
There is a misconception in the scientific community that van der Waals forces are weak forces, which needs some qualifications. It is well-known that van der Waals forces are short range forces, but can be extremely strong in magnitude if the separation between two objects (e.g., two atoms or molecules) is very small, say <0.4 nm. However, the magnitude of van der Waals forces drops precipitously when the separation increases even only slightly. Since the inter-graphene plane distance in un-intercalated and un-oxidized graphite crystal is small (<0.34 nm), the inter-graphene bonds (van der Waals forces) are actually very strong.
With an intercalation or oxidation treatment, the inter-graphene spacing is increased to a value typically greater than 0.55-0.65 nm. This is the first expansion stage experienced by the graphite material. The van der Waals forces are now significantly weakened due to the increased spacing. It is important to note that, in most cases, some of the graphene layers in a GIC are intercalated (with inter-graphene spacing increased to 0.55-0.65 nm and van der Waals forces weakened), but other layers could remain un-intercalated or incompletely intercalated (with inter-graphene spacing remaining approximately 0.34 nm and van der Waals forces staying strong).
In the conventional processes, the obtained GIC or GO, dispersed in the intercalant solution, will need to be rinsed for several cycles and then dried to obtain GIC or GO powders. These dried powders, commonly referred to as expandable graphite, are then subjected to further expansion or second expansion (often referred to as exfoliation) typically using a thermal shock exposure approach (at a temperature from 650° C. to 1,100° C.). The acid molecules residing in the inter-graphene spacing are decomposed at such a high temperature, generating volatile gas molecules that could push apart graphene planes.
Unfortunately, typically a significant proportion of the gaseous molecules escape without contributing to exfoliation of graphite flakes. Further, those un-intercalated and incompletely intercalated graphite layers remain intact (still having an inter-graphene spacing of approximately <0.34 nm). Additionally, many of the exfoliated flakes re-stack together by re-forming van der Waals forces if they could not be rapidly separated. These effects during this exfoliation step led to the formation of exfoliated graphite (referred to as Product (C) in FIG. 1), which is commonly referred to as “graphite worm” in the industry.
The exfoliated graphite or graphite worm is characterized by having networks of interconnected (un-separated) flakes which are typically >50 nm thick (often >100 nm thick). These individual flakes are each composed of hundreds of layers with inter-layer spacing of approximately 0.34 nm (not 0.6 nm), as evidenced by the X-ray diffraction data readily available in the open literature. In other words, these flakes, if separated, are individual graphite particles, rather than graphite intercalation compound (GIC) particles. This thermal shock procedure can produce some isolated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Again, the inter-flake distance between two loosely connected flakes or platelets is now increased to from 20 nm to several μm and, hence, the van der Waals forces that hold them together are now very weak, enabling easy separation by mechanical shearing or ultrasonication.
Typically, the exfoliated graphite or graphite worm is then subjected to a flake separation treatment using air milling, mechanical shearing, or ultrasonication in a liquid (e.g., water). Hence, a conventional process basically entails three distinct procedures: first expansion (oxidation or intercalation), further expansion (so called “exfoliation”), and separation. The resulting NGPs are graphene oxide (GO), rather than pristine graphene.
The work reported by Prud'Homme, et al. belongs to this category: R. K. Prud'Homme, et al. “Thermally Exfoliated Graphite Oxide,” US Pub. No. 2007/0092432 (Apr. 26, 2007) and “Tire Containing Thermally Exfoliated Graphite Oxide,” US Pub. No. 2009/0054581 (Feb. 26, 2009). In these two patent applications, natural graphite particles were heavily oxidized, to the extent that the characteristic inter-graphene plane distance of 0.335 nm associated with natural graphite was no longer observable using X-ray diffraction. The resulting exfoliated graphite platelets were heavily oxidized graphite or graphite oxide, as clearly indicated by the title of the patent applications.
D. M. Kaschak, et al. [“Graphite Intercalation and Exfoliation Process,” US Pub. No. 2004/0033189 (Feb. 19, 2004); and “Graphite Composites and Method of Making Such Composites,” U.S. Pat. No. 6,927,250, Aug. 9, 2005] proposed a method of modifying graphite by introducing a supercritical fluid into interstices of chemically intercalated or intercalated/oxidized graphite (rather than the original natural graphite). The interstices of intercalated and/or oxidized graphite had been expanded and chemically modified due to the presence of intercalant species (such as sulfuric acid) or oxidation-induced functional groups (such as carboxyl). Kaschak, et al. did not teach about the approach of directly intercalating the un-treated natural flake graphite with a supercritical fluid. The modified graphite as proposed by Kaschak, et al. still required a high temperature exposure step, typically at 700-1,200° C., to exfoliate the intercalated and modified graphite. The products were graphite oxide flakes, not pristine graphene.
Furthermore, Kaschak, et al. did not provide any evidence to show the existence of nano-scaled graphite particles that they claimed they produced with this method. In particular, they claimed that “one advantage of the invention is that the aforementioned methods may be used to manufacture graphite in a form that has a thickness of less than about 10 microns, preferably less than about 1 micron, more preferably less than about 100 nm, even more preferably less than about 10 nm, and most preferably less than about 1 nm.” However, they did not fairly suggest the conditions under which graphite particles with a thickness less than 10 nm or 1 nm could be produced. This was truly a broad and aggressive claim and should have been supported by solid experimental evidence; unfortunately, absolutely no evidence whatsoever was presented.
In the conventional processes, the post-exfoliation ultrasonication procedure was meant to break up graphite worms (i.e., to separate those already largely expanded/exfoliated flakes that are only loosely connected). Specifically, it is important to emphasize the fact that, in the prior art processes, ultrasonification is used after intercalation and oxidation of graphite (i.e., after first expansion) and most typically after thermal shock exposure of the resulting GIC or GO (i.e., after second expansion or exfoliation) to aid in breaking up those graphite worms. There are already much larger spacings between flakes after intercalation and/or exfoliation (hence, making it possible to easily separate flakes by ultrasonic waves). This ultrasonication was not perceived to be capable of separating those un-intercalated/un-oxidized layers where the inter-graphene spacing remains <0.34 nm and the van der Waals forces remain strong.
The applicant's research group was the very first in the world to surprisingly observe that, under proper conditions (e.g., with the assistance of a surfactant and using a higher sonic power), ultrasonication is capable of producing ultra-thin, pristine graphene directly from pristine graphite, without having to go through chemical intercalation or oxidation. This invention was reported in a patent application [A. Zhamu, J. Shi, J. Guo, and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano Graphene Plates,” U.S. patent Ser. No. 11/800,728 (May 8, 2007)].
Schematically shown in FIG. 2 are the essential procedures used to produce single-layer or few-layer graphene using this direct ultrasonication process. This innovative process involves simply dispersing pristine graphite powder particles in a liquid medium (e.g., water, alcohol, or acetone) containing a dispersing agent or surfactant to obtain a suspension. The suspension is then subjected to an ultrasonication treatment, typically at a temperature between 0° C. and 100° C. for 10-120 minutes. No chemical intercalation or oxidation is required of the starting material prior to ultrasonication. The graphite material has never been exposed to any obnoxious chemical throughout the entire nano graphene production process. This process combines expansion, exfoliation, and separation of pristine graphitic material into one step. Hence, this simple yet elegant method obviates the need to expose graphite to a high-temperature, or chemical oxidizing environment. The resulting NGPs are essentially pristine graphene, which is highly conductive both electrically and thermally.
In the scientific community and in nano materials industry, NGPs are considered a new class of nano materials that is different and distinct from fullerene, carbon nanotubes (CNTs), and graphite nanoparticles for the following main reasons:                (a) Fullerene is considered a zero-dimensional carbon nano material due to its ultra-small sizes in all directions.        (b) CNTs are considered a type of one-dimensional carbon nano material due to their large size in one dimension (length), but small size in other two dimensions (cylindrical cross-section with a diameter <100 nm, more typically <30 nm, and, for single-walled CNTs, <1.0 nm).        (c) Graphite particles (including both micron-scaled and nano-scaled) are considered a three-dimensional carbon material since they have substantially identical or similar sizes in all three directions (X—, Y—, and Z-coordinates). Most of the conventional graphite nanoparticles are close to being spherical or ellipsoidal in shape having a diameter less than 500 nm, but typically >350 nm. Graphite nano particles are produced simply by pulverizing or grinding and then ball-milling natural graphite particles from typically greater than 100 μm to sub-micron in diameter (typically <500 nm, but >>100 nm). In real practice, it is difficult to grind and mill graphite particles down to a size smaller than 350 nm.        (d) NGPs are considered a two-dimensional carbon nano material with large sizes in two dimensions (both length and width typically >0.5 μm, but more typically >1 μm) and ultra-small in one dimension (thickness as small as one carbon atom size).Due to these differences in geometry, these four classes of carbon materials also exhibit vastly different properties. For instance, the graphite nano particles were normally viewed as excellent thermally conducting materials with a high thermal conductivity of up to 60-80 W/m−k. However, this conductivity value range is almost two orders of magnitude lower than the thermal conductivity of NGPs, just recently found to be as high as 5,300 W/m−k. Thermal conductivity of carbon black, also considered a type of carbon nano particle, is even lower.        
In order for NGPs (either pristine graphene or graphene oxide) to be an effective nano-filler for a polymer composite, NGPs must be able to form a stable, uniform dispersion in the polymer matrix. In other words, proper dispersion of NGPs in a polymer would be a prerequisite to achieving good thermal and dynamic (friction and wear) properties of the resulting rubbery composite materials. These issues have not been addressed and the potential of using these highest-performing NGPs as an additive for a tire has not been explored.
It is therefore an object of the present invention to provide a cost-effective tire rubber composition that exhibits superior anti-wear, wet-grip, and thermal conductivity properties.
It is yet another object of the present invention to provide a pristine NGP-containing tire that exhibits improved heat transfer properties as compared to corresponding tires containing graphite oxide nano platelets or graphite nano particles.
Still another object of the present invention is to provide a rubbery tire that exhibits a better combination of friction, wear, and heat transfer properties as compared with a corresponding rubber composition containing silica or carbon black nano particles.